The United States Federal Railroad Administration has published statistics which indicate that train accidents caused by track failures including rail, joint bar and anchoring resulted in approximately 1,300 derailments from 2001 to 2011. The primary cause of these track failures was defects and fissures in the rail head.
During their normal use and as would be expected, the rail portions of most track structures will be subjected to severe, and uncontrollable environmental conditions. These severe environmental conditions, over a relatively long period of time, may ultimately result in such rail developing certain detrimental flaws.
In addition, in today's modern railroad industry, the rail portion of such track structures will quite often be required to support rather heavy loads being carried by modern freight cars. Furthermore, these heavy loads are travelling at relatively high speeds. It would not be uncommon for these freight cars, when they are fully loaded with cargo, to weigh up to generally about 125 tons. Such relatively heavy loads and high speeds can, also, result in undesirable damage to such rail portions of the track structure. Such damage, for example, may include stress fractures.
It would be expected, therefore, that if these detrimental defects were not timely detected and, likewise, if they are left unrepaired such defects could lead to some rather catastrophic disasters, such as, a train derailment.
As is equally well known, such train derailments are not only costly to the railroad industry from the standpoint of the damage that will likely be incurred to both the cargo being transported and to the railway equipment itself, but, even more importantly, such train derailments may also involve some rather serious injuries, or even worse death, to railway personnel and/or other persons who may be in the vicinity of a train derailment.
It is further well known that a relatively large number of these train derailments have resulted in the undesirable and often costly evacuation of nearby homes and businesses. Such evacuation may be required, for example, when the cargo being transported involves certain highly hazardous chemical products. These hazardous chemical products will generally include both certain types of liquids, such as corrosive acids, and certain types of toxic gases, such as chlorine.
To detect such flaws and defects, ultrasonic testing has been employed. Vehicles have been built which travel along the track and continuously perform ultrasonic testing of the track. These vehicles carry test units which apply ultrasonic signals to the rails, receive ultrasonic signals back from the rails, and provide indications of flaws and defects.
Some of these systems employ small, thin-walled tires which roll along the rails. They are pressed down against the rail so as to have a flat area in contact with the rail. These tires contain acoustic transducers and are filled with a liquid, usually a water-glycol solution. The transducers are arranged at various angles to produce acoustic beams which travel through the mounting substrate and liquid and are directed toward the rail surface. The angles are predetermined based on the known geometry of a new rail. The high frequency electrical transducers are pulsed with energy and the generated beams pass through the material of the liquid and tire into the rail. The angle of incident of the beam with respect to the rail surface is predetermined based on the desired angle of refraction in a known material, assuming a horizontal head shape according to Snell's law.
Only a few transducers can be mounted to the substrate due to spatial considerations. Also, the angles of the acoustic beams produced by the transducers are dictated by their fixed mounting angle. The rail head may be worn or deformed by the massive loads and stresses to which it is subjected. The shape of the rail head may change over time whereby the running surface of the rail head is no longer substantially horizontal. Because many of the inspection systems employ ultrasonic transducers mounted in a fixed position at a fixed angle relative to a presumed horizontal inspection surface, the resulting beam inspection angles may not be optimal and may fail to detect defects in the rail.